Building Java Programs Chapter 5 Lecture 11: while Loops, Fencepost - - PowerPoint PPT Presentation

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Building Java Programs

Chapter 5 Lecture 11: while Loops, Fencepost Loops, and Sentinel Loops reading: 5.1 – 5.2

(Slides adapted from Stuart Reges, Hélène Martin, and Marty Stepp)

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Methods using charAt

— Write a method printConsonants that accepts a String as

a parameter and prints out that String with all vowels removed For example, the call:

printConsonants("atmosphere")

should print:

tmsphr

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A deceptive problem...

— Write a method printLetters that prints each letter from

a word separated by commas. For example, the call:

printLetters("Atmosphere")

should print:

A, t, m, o, s, p, h, e, r, e

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Flawed solutions

— public static void printLetters(String word) {

for(int i = 0; i < word.length(); i++) { System.out.print(word.charAt(i) + ", "); } System.out.println(); // end line }

— Output:

A, t, m, o, s, p, h, e, r, e,

— public static void printLetters(String word) {

for(int i = 0; i < word.length(); i++) { System.out.print(", " + word.charAt(i)); } System.out.println(); // end line }

— Output:

, A, t, m, o, s, p, h, e, r, e

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Fence post analogy

— We print n letters but need only n - 1 commas. — Similar to building a fence with wires separated by posts:

— If we use a flawed algorithm that repeatedly places a post +

wire, the last post will have an extra dangling wire. for (length of fence) { place a post. place some wire. }

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Fencepost loop

— Add a statement outside the loop to place the initial "post."

— Also called a fencepost loop or a "loop-and-a-half" solution.

place a post. for (length of fence - 1) { place some wire. place a post. }

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Fencepost method solution

— public static void printLetters(String word) {

System.out.print(word.charAt(0)); for(int i = 1; i < word.length(); i++) { System.out.print(", " + word.charAt(i)); } System.out.println(); // end line }

— Alternate solution: Either first or last "post" can be taken out:

public static void printLetters(String word) { for(int i = 0; i < word.length() - 1; i++) { System.out.print(word.charAt(i) + ", "); } int last = word.length() – 1; System.out.println(word.charAt(last)); // end line }

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Fencepost question

— Write a method printPrimes that prints all prime numbers

up to a max.

— Example: printPrimes(50) prints

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47

— If the maximum is less than 2, print no output.

— To help you, write a method countFactors which returns

the number of factors of a given integer.

— countFactors(20) returns 6 due to factors 1, 2, 4, 5, 10, 20.

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Fencepost answer

// Prints all prime numbers up to the given max. public static void printPrimes(int max) { if (max >= 2) { System.out.print("2"); for (int i = 3; i <= max; i++) { if (countFactors(i) == 2) { System.out.print(", " + i); } } System.out.println(); } } // Returns how many factors the given number has. public static int countFactors(int number) { int count = 0; for (int i = 1; i <= number; i++) { if (number % i == 0) { count++; // i is a factor of number } } return count; }

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while loops

reading: 5.1

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Categories of loops

— definite loop: Executes a known number of times.

— The for loops we have seen are definite loops.

— Print "hello" 10 times. — Find all the prime numbers up to an integer n. — Print each odd number between 5 and 127.

— indefinite loop: One where the number of times its body

repeats is not known in advance.

— Prompt the user until they type a non-negative number. — Print random numbers until a prime number is printed. — Repeat until the user has typed "q" to quit.

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The while loop

— while loop: Repeatedly executes its

body as long as a logical test is true.

while (test) { statement(s); }

— Example:

int num = 1; // initialization while (num <= 200) { // test System.out.print(num + " "); num = num * 2; // update } // output: 1 2 4 8 16 32 64 128

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Example while loop

// finds the first factor of 91, other than 1 int n = 91; int factor = 2; while (n % factor != 0) { factor++; } System.out.println("First factor is " + factor); // output: First factor is 7

— while is better than for because we don't know how many

times we will need to increment to find the factor.

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— sentinel: A value that signals the end of user input.

— sentinel loop: Repeats until a sentinel value is seen.

— Example: Write a program that prompts the user for text

until the user types "quit", then output the total number of characters typed.

— (In this case, "quit" is the sentinel value.)

Type a word (or "quit" to exit): hello Type a word (or "quit" to exit): yay Type a word (or "quit" to exit): quit You typed a total of 8 characters.

Sentinel values

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Solution?

Scanner console = new Scanner(System.in); int sum = 0; String response = "dummy"; // "dummy" value, anything but "quit" while (!response.equals("quit")) { System.out.print("Type a word (or \"quit\" to exit): "); response = console.next(); sum += response.length(); } System.out.println("You typed a total of " + sum + " characters.");

— This solution produces the wrong output. Why?

You typed a total of 12 characters.

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The problem with our code

— Our code uses a pattern like this:

sum = 0. while (input is not the sentinel) { prompt for input; read input. add input length to the sum. }

— On the last pass, the sentinel’s length (4) is added to the

sum:

prompt for input; read input ("quit"). add input length (4) to the sum.

— This is a fencepost problem.

— Must read N lines, but only sum the lengths of the first N-1.

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A fencepost solution

sum = 0. prompt for input; read input. // place a "post" while (input is not the sentinel) { add input length to the sum. // place a "wire" prompt for input; read input. // place a "post" }

— Sentinel loops often utilize a fencepost "loop-and-a-half"

style solution by pulling some code out of the loop.

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Correct code

Scanner console = new Scanner(System.in); int sum = 0; // pull one prompt/read ("post") out of the loop System.out.print("Type a word (or \"quit\" to exit): "); String response = console.next(); while (!response.equals("quit")) { sum += response.length(); // moved to top of loop System.out.print("Type a word (or \"quit\" to exit): "); response = console.next(); } System.out.println("You typed a total of " + sum + " characters.");

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Sentinel as a constant

public static final String SENTINEL = "quit"; ... Scanner console = new Scanner(System.in); int sum = 0; // pull one prompt/read ("post") out of the loop System.out.print("Type a word (or \"" + SENTINEL + "\" to exit): "); String response = console.next(); while (!response.equals(SENTINEL)) { sum += response.length(); // moved to top of loop System.out.print("Type a word (or \"" + SENTINEL + "\" to exit): "); response = console.next(); } System.out.println("You typed a total of " + sum + " characters.");